Polyimide membranes derived from 6FDA-DAM:DABA and 6FDA-6FpDA:DABA copolymers have been used to separate 50/50 CO2/CH4 mixtures and multicomponent synthetic natural gas mixtures at 35 degreesC and feed pressures up to 55 atm. For 6FDA-DAM:DABA 2:1 membranes the effects of thermal annealing and covalent crosslinking are decoupled with respect to effects on permeabilities and selectivity. Crosslinking at 295 degreesC with 1,4-butylene glycol and 1,4-cyclohexanedimethanol increases CO2 permeabilities by factors of 4.1 and 2.4, respectively, at 20 atm feed pressure, without a loss in selectivity, relative to crosslinking at 220 degreesC. Thermal annealing and crosslinking also reduce CO2 plasticization effects. Crosslinking of DABA-containing copolymers, therefore, can produce membranes with tunable transport properties that offer significantly higher performance with better plasticization-resistance than that reported in the literature for the commercial polymers Matrimid(R) and cellulose acetate for CO2 removal from natural gas mixtures. Separation of complex mixtures containing CO2, CH4, C2H6, C3H8, and C4H10 or toluene results in a significant decrease of the CO2 permeability, but only a moderate decrease in the CO2/CH4 selectivity. (C) 2003 Elsevier B.V. All rights reserved.